Protein kinases play a critical role in mediating signaling events leading to cellular responses such as activation, growth and differentiation, in response to extracellular signals. Protein kinases transmit their signal by phosphorylating specific residues in a target protein. Protein kinases that specifically phosphorylate tyrosine residues are referred to as protein tyrosine kinases. Protein tyrosine kinases can be divided into two general groups: receptor such as epidermal growth factor (EGF) receptor (S. Iwashita and M. Kobayashi, 1992, Cellular Signalling, 4, 123–132) and cytosolic non-receptor (C. Chan et al., 1994, Ann. Rev. Immunol., 12, 555–592).
Interleukin-2-inducible T cell kinase (Itk), also referred to as T cell-specific kinase (Tsk) and expressed mainly in T-lymphocytes (EMT), is a member of the Tec family of protein tyrosine kinases that also includes Txk, Tec, Btk, and Bmx. Tec family members are characterized by the presence of a pleckstrin-homology domain (PH), a proline rich Tec homology domain (TH) and Src homology SH3, SH2 and SH1 kinase domains positioned from the N-terminus to the C-terminus respectively (S. Gibson et al., 1993, Blood, 82,1561–1572; J. D. Siliciano et al., 1992, Proc. Nat. Acad. Sci., 89, 11194–11198; N. Yamada et al., 1993 Biochem. and Biophys Res. Comm., 192, 231–240).
Itk is expressed in T cells, mast cells and natural killer cells. It is activated in T cells upon stimulation of the T cell receptor (TCR), and in mast cells upon activation of the high affinity IgE receptor. Following receptor stimulation in T cells, Lck, a src tyrosine kinase family member, phosphorylates Y511 in the kinase domain activation loop of Itk (S. D. Heyeck et al., 1997, J. Biol. Chem, 272, 25401–25408). Activated Itk, together with Zap-70 is required for phosphorylation and activation of PLC-γ (S. C. Bunnell et al., 2000, J. Biol. Chem., 275, 2219–2230). PLC-γ catalyzes the formation of inositol 1,4,5-triphosphate and diacylglycerol, leading to calcium mobilization and PKC activation, respectively. These events activate numerous downstream pathways and lead ultimately to degranulation (mast cells) and cytokine gene expression (T cells) (Y. Kawakami et al., 1999, J. Leukocyte Biol., 65, 286–290).
The role of Itk in T cell activation has been confirmed in Itk knockout mice. CD4+T cells from Itk knockout mice have a diminished proliferative response in a mixed lymphocyte reaction or upon Con A or anti-CD3 stimulation. (X. C. Liao and D. R. Littman, 1995, Immunity, 3, 757–769). Also, T cells from Itk knockout mice produced little IL-2 upon TCR stimulation resulting in reduced proliferation of these cells. In another study, Itk deficient CD4+ T cells produced reduced levels of cytokines including IL-4, IL-5 and IL-13 upon stimulation of the TCR, even after priming with inducing conditions. (D. J. Fowell, 1999, Immunity, 11, 399–409).
The role of Itk in PLC-γ activation and in calcium mobilization was also confirmed in the T cells of these knockout mice, which had severely impaired IP3 generation and no extracellular calcium influx upon TCR stimulation (K. Liu et al., 1998, J. Exp. Med. 187, 1721–1727). The studies described above support a key role for Itk in activation of T cells and mast cells. Thus an inhibitor of Itk would be of therapeutic benefit in diseases mediated by inappropriate activation of these cells.
It has been well established that T cells play an important role in regulating the immune response (Powrie and Coffman, 1993, Immunology Today, 14, 270–274). Indeed, activation of T cells is often the initiating event in immunological disorders. Following activation of the TCR, there is an influx of calcium that is required for T cell activation. Upon activation, T cells produce cytokines, including IL-2, 4, 5, 9, 10, and 13 leading to T cell proliferation, differentiation, and effector function. Clinical studies with inhibitors of IL-2 have shown that interference with T cell activation and proliferation effectively suppresses immune response in vivo (Waldmann, 1993, Immunology Today, 14, 264–270). Accordingly, agents that inhibit T lymphocyte activation and subsequent cytokine production, are therapeutically useful for selectively suppressing the immune response in a patient in need of such immunosuppression.
Mast cells play a critical roll in asthma and allergic disorders by releasing pro-inflammatory mediators and cytokines. Antigen-mediated aggregation of FcεRI, the high-affinity receptor for IgE results in activation of mast cells (D. B. Corry et al., 1999, Nature, 402, B18–23). This triggers a series of signaling events resulting in the release of mediators, including histamine, proteases, leukotrienes and cytokines (J. R. Gordon et al., 1990, Immunology Today, 11, 458–464.) These mediators cause increased vascular permeability, mucus production, bronchoconstriction, tissue degradation and inflammation thus playing key roles in the etiology and symptoms of asthma and allergic disorders.
Recent published data using Itk knockout mice suggests that in the absence of Itk function, increased numbers of memory T cells are generated (A. T. Miller et al., 2002 The Journal of Immunology, 168, 2163–2172). One strategy to improve vaccination methods is to increase the number of memory T cells generated (S. M. Kaech et al., Nature Reviews Immunology, 2, 251–262).
All documents cited in this application are incorporated by reference in their entirety.